Since the start of electroplating, a large number of proposals have been made with respect to electroplating on non-electrically-conductive substrates ranging in size and shape across the gamut of leaves, flowers, baby shoes, plastic knobs, bottle tops, molded plastic parts for automotive usage and uncounted other practical and decorative structures. Basically, two processes have been used. The first process involves the coating of the non-conductive object with an electrically conductive lacquer followed by electroplating. The second process involves sensitizing the non-conductive object, chemically depositing a metal on the sensitized surface and thereafter electroplating the thus metallized surface.
The two generally available processes as practiced in the prior art have certain disadvantages. Because of high loadings of conductive pigments such as graphite or metal, prior art conductive lacquers are generally very weak and thus constitute a weak link in the ultimate electroplated structure. A variation of the lacquer process which involves coating the tacky lacquer surface with graphite again produces very weak bonds between electrodeposited metal and the lacquer much like the ephemeral bond produced between graphitized wax and electrodeposited metal in the electrotyping process. If lower pigment loadings are used in a conductive lacquer to give greater strength in the lacquer, the rate of initial metal coverage of the article during electroplating is radically decreased necessitating the use of multiple electrical contact points on the object to be plated or allowance of a long time for metal coverage and consequent uneven plating thicknesses.
The second process as generally practiced by the prior art, can achieve good results but only at a cost of employing a large number of individual processing operations carried out with very great care by skilled personnel. Furthermore, because the underlying chemically deposited metal can be different from metal subsequently electrochemically deposited, there is a good chance of forming an electrochemical couple between the two even when, nominally the metals are the same. Thus the possibility of accelerated, localized corrosion exists wherever and whenever the outer electrodeposited layer is not continuous.
Recently, U.S. Pat. No. 3,523,875 to Minklei and No, 3,682,786 to Brown et al have issued. These recently issued patents are worthy of discussion because, superficially they might appear to resemble the process of the present invention. Minklei proposed to treat a plastic surface with an aqueous solution of alkali metal sulfide followed by contacting the treated surface with a metal salt prior to electroplating. Brown et al proposed contacting a plastic surface with a solution or dispersion of sulfur in an organic medium and contacting the treated surface with an aqueous solution of cuprous salt prior to plating. In both instances, the proposals involve the formation of a metal sulfide on the plastic surface and not the type of metal-polymer bond, which, as will become apparent from the subsequent description, is formed by virtue of the process of the present invention.